How each one carries load
A stone column is a column of compacted crushed stone. It has no strength of its own: it relies on the surrounding soil to hold it in shape. Load is shared between the columns and the soil, and the columns also act as drains that speed up consolidation.
A rigid inclusion is a column of grout or concrete. It is much stiffer than the soil and does not depend on the soil for confinement. Most of the load passes down the inclusions to a firmer layer, through a compacted granular layer called a load transfer platform.
Side-by-side comparison
| Factor | Stone columns | Rigid inclusions |
|---|---|---|
| Material | Compacted crushed stone | Grout or concrete, usually unreinforced |
| Relies on the soil for confinement | Yes | No |
| Very soft clay or peat | Limited: columns can bulge | Can be considered |
| Drainage | Columns drain the soil | No drainage |
| Liquefaction mitigation | Can densify and drain suitable soils | Not its usual purpose |
| Settlement control | Reduces settlement | Tighter settlement control |
| Connection to the structure | None; granular blanket on top | None; load transfer platform on top |
When stone columns are usually considered
- Soft to firm silts and clays, or mixed soils, that can confine the stone.
- Loose silty sands where both densification and drainage help.
- Embankments, tanks and warehouses that can tolerate some settlement.
- Sites where faster consolidation is useful to the programme.
When rigid inclusions are usually considered
- Very soft or organic soils that cannot confine a stone column.
- Structures with tight total or differential settlement limits.
- Heavy slab or tank loads over deep compressible layers.
- Sites where a suitable stone supply is difficult.
Rigid inclusions are not piles. They are not connected to the structure, and the design of the load transfer platform is as important as the inclusions themselves.
How each is verified
Stone columns are checked through installation records for each column (depth, stone consumption and compaction effort), followed by plate or zone load tests and, in suitable soils, CPT between columns.
Rigid inclusions are checked through installation records (depth, grout or concrete volume and pressure), integrity tests, load tests on single inclusions and tests on the load transfer platform.